Automatic focus of an optical system requires the acquisition of information about the relative position of the object and the optical system. In many instances, the object approximates to a plane reflective surface, and the auto-focus will project a light beam onto the object and use the zero-order reflection from the object to determine the object distance.
Although such systems have been deployed with some success, they have suffered from a variety of disadvantages including: 1) that surface detail in localized regions of the object will affect the zero-order reflection and in some implementations will result in false readings; 2) that the light beam used for focus investigation may not have the same chromatic properties as the light which is used by the optical system when it is performing its intended task; 3) that the optical path of the system used for focus investigation may differ substantially from that used by the optical system when it is performing its intended task; 4) that such systems may employ a variety of artifacts which are not a part of the object under investigation, some of which may contribute to false readings.
The present invention may best be described in the context of its application with a bright-field microscope as used for examining silicon wafers for the purpose of process control. A particularly preferred application of the invention is for measurement of focus information in overlay metrology in which the focal conditions under which the data are gathered have a substantial impact on the quality of the data and this example is discussed in detail herein. Potentially however, the present invention could be used for any optical system in which there is a spatially-concentrated light-source.
In overlay metrology, light is injected into the top focal plane of a microscope objective to illuminate the object. Light reflected from that object is collected by the same objective and directed by means of a beam-splitter through an optical system to an imaging system which forms an image of the object. Typically this will comprise an array detector or detector such as a CCD camera. The object consists of a pair of marks produced by photolithography on a silicon wafer. Overlay metrology is the process whereby the relative positions of the two marks are measured. Historically, these marks have tended to be marks with four-way rotational symmetry which are positioned so that they are nominally concentric. One mark is larger than the other so that the two marks may easily be distinguished. They are referred to as the inner mark and the outer mark. Overlay marks generally have straight edges.
For the purpose of discussions herein, light used for gathering focus data will be referred to as the focus beam and light which is used when the optical system is performing its intended task (e.g. overlay metrology) will be referred to as the metrology beam.
One method by which the correct focal distance for an object may be determined is to gradually change the object distance while continuously gathering data from the image formed by the optical system. If there is a well-defined criterion by which the “best focus” position can be judged (e.g. maximum spatial frequency content of the image, maximum intensity gradient, etc.) then the data collected as the object distance varies may be analysed to determine at which focal distance the defined criterion is best complied with. Following this, the focus distance may be set to the identified best focus condition and the optical system can be used for metrology. Alternatively, if sufficient data were acquired during the through-focus scan, those data which were acquired in the out-of-focus condition may be discarded while those that were gathered at the in-focus condition will be used for metrology.
This methodology requires that a lot of data are acquired and analysed and is inevitably slow as time is taken up gathering data which are later discarded. To avoid this, many attempts have been made to develop auto-focus systems and auto-focus methods in which focus data may be acquired much more rapidly.
In many auto-focus systems, light beams are injected into the optical system by means of a beam-splitter. These injected beams will emerge from the objective, reflect from the surface of the object and return to the optical system. The injected beams have some character which may be measured in the returning beam and which will be modified by a change in the distance of the object.
These methods can give focus information within a much shorter time interval. Systems employing this principle include laser-spot focus systems, twin aperture measurement systems and astigmatic beam systems. Such auto-focus systems may work well in a range of conditions, but are subject to a number of practical limitations.
First, these systems generally assume that the object is a minor normal to the optical axis, and sample a limited region (or in some cases regions) of the object under investigation. Localized topography of the sample can result in false readings. Reflectance variation of the sample, etc. can cause degradation of the focus information that is obtained, as it changes the character of the light beam that is being measured. There are many situations in which these degradations are not present or are negligible, and some of these auto-focus systems work very well within a limited context. However, in the case of overlay metrology the requirement to achieve extreme focus sensitivity necessitates elimination of as many potential sources of uncertainty as possible.
Second, the focus beam may be very limited in the wavelengths that it can use, e.g. a laser spot focus system will usually be limited to a single wavelength. The light in the metrology beam may be from a broad band source. In any system in which chromatic aberration had not been perfectly eliminated (i.e. any refractive optical system) there would be some offset between the best focus determined using the focus beam and the focus required by the metrology beam. As there may be some chromatic filtration of the metrology beam by the object (e.g. thin-film filtration on the surface of a silicon wafer) the offset may vary from sample to sample and may not be known.
It would therefore be advantageous to use light with the same chromatic character for the focus beam as for the metrology beam.